Mature Oil Paint Systems
Many studies undertaken in the past decade have demonstrated that over time comparable chemical and physical processes take place in all oil paints, despite large variations in composition and history. However there still is no thorough understanding of the reaction rates and the degree to which internal and external factors affect ageing and deterioration processes in oil paints.
Traditional oil paint is mostly a heterogeneous, multi-layered system involving mixtures of different pigments (and additives) bound in an organic medium that has been subjected to different environmental conditions and conservation treatments.
Chemically, drying oils are made of poly-unsaturated triglyceride moieties (TAGs) - esters of glycerol with long-chain fatty acids with one or more unsaturated bonds. The curing of oil paint is a complex process, which involves cross-linking of the triglyceride moieties and metal-oil interactions as well as degradation reactions that continue as the paint ages such as [Van den Berg 2002; Boon 2007; Tumosa et al. 2005]:
Hydrolysis of the original ester bonds of the cross-linked triacylglyceride moieties may result in the formation of free carboxylic groups within the network that may be stabilized by metal interactions, or in the formation of free mono- and dicarboxylic fatty acids (palmitic (FA16) and stearic (FA18) acid; azelaic (diFA9) acid).
Oxidation and chain breaking processes may lead to acidic products such as dicarboxylic fatty acids and short-chain (volatile) products.
- Metal soap formation
Certain metal-based pigments (lead, zinc, copper or potassium-containing) may react with an excess of free fatty acids to form metal carboxylate fatty acid salts (metal soaps).
A mature oil paint micro-structure should clearly be considered as a semi-permeable system where transport of the mobile fraction including free fatty acids and metal soaps can take place through interparticle and low-density regions or through nano-pores and micro-cracks.
These are all competing reactions. It depends largely on the internal and external conditions in the paint as to which reactions will prevail and what the resulting physical film properties will be.
Mature oil paint consists of a stationary fraction of metal-coordinated 3D network fragments and a mobile fraction with free mono- and diacids, metal soaps, water-soluble inorganic components, water and small volatile oxidation products. These fractions are not static in composition, but are constantly changing as a result of chemical and physical processes.
Earlier studies have shown, for example, that the process of hydrolysis increases in time. Sutherland demonstrated that extracts from 17th to 19th-century paintings contain even more free fatty acids than young model systems [Sutherland 2001]. In another case, bulk measurements of a lead white/chalk ground in a 19th-century painting by Church showed 80% hydrolysis [Van den Berg 2001].
Lack of understanding
In general there is no thorough understanding of the reaction rates and the degree to which internal and external factors affect ageing and deterioration processes in oil paints. For example over the last fifteen years it has been recognized in many publications that pigment-medium interactions and the formation of metal soaps play an important part in many of the commonly observed degradation phenomena.
In these publications the effects on the visual characteristics of the paintings has been described [Noble et al. 2000; Higgitt et al 2003; Spring et al. 2005; Keune and Boon 2007; Van Loon 2008; Noble et al. 2008]. However, how to deal with metal soaps during conservation treatment is much less straightforward. Little is known about the properties of metal soaps inside paint layers and the influence of the various internal and external factors on their development in time.